1. Field of the Invention
The present invention relates to a fiber coupler for coupling light into an optical fiber and/or coupling light out of the fiber. The fiber coupler has two holder elements, namely a fiber holder for holding the fiber, and a lens holder for holding a focusing lens, whereby the holder elements are adjustable in the axial and transversal direction, respectively. Such fiber couplers are needed, among other things, in the sector of research, for optical experiments, and in the field of telecommunications technology, for systems for optical signal transmission.
2. The Prior Art
High requirements with regard to the precision of the positioning of the fiber end relative to the focusing lens are set for fiber couplers for coupling a free light beam into an optical fiber and for coupling light out of the fiber. Usual so-called single mode fibers have a mode field diameter of only a few micrometers. In the case of single mode fibers for a light wavelength of 780 nm, for example, the mode field diameter is only 4 μm. To couple a free light beam into such a fiber, it must therefore be possible to position the focusing lens to a precision of a few micrometers, along the optical axis. Only in this way can optimal efficiency be achieved when coupling in. A tolerance of 10 μm already reduces the efficiency of the coupling by 10%. The efficiency of the coupling is even more sensitively dependent on the transversal positioning relative to the fiber end. A deviation from the optimal position of only one micrometer in the transversal direction reduces the efficiency of the coupling by 20%.
A fiber coupler of the type stated initially is previously known from U.S. Pat. No. 5,282,393 A, for example. In the case of the previously known coupler, transversal and axial setting members, respectively, are provided to adjust the holder elements for the fiber and the lens, respectively. These members essentially consist of threaded rods that are guided in corresponding threaded bores of a base element. In this previously known coupler, the setting member provided for axial adjustment of the focusing lens along the optical axis consists of three threaded rods having screws guided on them. These screws are synchronously turned by means of a gear mechanism, in order to thereby allow linear movement of the focusing lens along the optical axis.
It is a disadvantage of the aforementioned fiber coupler that it consists of a plurality of mechanical components for the various setting members, which must be produced with the greatest precision. For this reason, the previously known fiber coupler is extremely expensive in its production. It is a further disadvantage that the gear mechanism described above can hardly be produced with sufficient precision for the axial adjustment of the focusing lens, in order to satisfy the requirements for fiber couplers used with single mode fibers, as mentioned above. Another disadvantage results from the complicated mechanical design of the previously known fiber coupler. It is a disadvantage that the structure of the coupler is not very compact. Finally, another disadvantage is the insufficient precision in the adjustment of the lens or the fiber end in the transversal direction, respectively, as well as the poor long-term stability of the adjustment.